Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. For example, Spinal Cord Stimulation (SCS) techniques, which directly stimulate the spinal cord tissue of the patient, have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of spinal cord stimulation has begun to expand to additional applications, such as angina pectoralis and incontinence.
An implantable SCS system typically includes one or more electrode carrying stimulation leads, which are implanted at a stimulation site in proximity to the spinal cord tissue of the patient, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Thus, programmed electrical pulses can be delivered from the neurostimulator to the stimulation lead(s) to stimulate or activate a volume of the spinal cord tissue. In particular, electrical stimulation energy conveyed to the electrodes creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neural fibers within the spinal cord beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers to provide the desired efficacious therapy to the patient.
For example, in the context of providing pain relief via SCS, it is believed that the antidromic activation (i.e., the APs propagate in a direction opposite to their normal direction, which in the case of the spinal cord, propagate in the caudal direction) of the spinal cord fibers provides the actual pain relief to the patient by reducing/blocking transmission of smaller diameter pain fibers via interneuronal interaction in the dorsal horn of the spinal cord, while the orthodromic activation (i.e., the APs propagate in their normal direction, which in the case of the spinal cord, propagate in the rostral direction) of the spinal cord fibers generate APs that arrive at the thalamus and are relayed to the sensory cortex, thereby creating a pleasant side-effect in the form of a sensation known as paresthesia, which can be characterized as an tingling sensation that replaces the pain signals sensed by the patient.
The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include electrical pulse parameters, which may define the pulse amplitude, pulse width, pulse rate, pulse shape, and burst rate. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen. The regimen will dictate which of the electrodes are sourcing current pulses (anodes) and which of the electrodes are sinking current pulses (cathodes) at any given time, as well as the magnitude and duration of the current pulses. The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated.
While the electrical stimulation of neurons has generally been successful in providing a therapeutic benefit to the patient, there are instances where the target tissue is not directly adjacent to an electrode and, because the electrical field strength decreases exponentially with distance from the electrodes, a relatively strong electrical field must be created to generate APs in the target neural fibers. The electrical field may, however, also result in the generation of APs in the non-target neural fibers, which are larger size than target fibers or closely located to the electrode. The generation of APs in the non-target neural fibers may, in turn, lead to undesirable outcomes (e.g., discomfort or involuntary movements) for the patient. Because the target neural tissue (i.e., the tissue associated with the therapeutic effects) and non-target neural tissue (i.e., the tissue associated with undesirable side effects) are often juxtaposed, therapeutically stimulating neural tissue while preventing side effects may be difficult to achieve.
For example, in the context of SCS, to provide pain relief without inducing involuntary motor movements or otherwise causing discomfort, the neural fibers in the dorsal column (DC neural fibers), which primarily include sensory neural fibers, may be preferentially stimulated over neural fibers in the dorsal roots (DR neural fibers), which, like the DC neural fibers, includes sensory neural fibers. However, stimulation of the DR neural fibers may also create a monosynaptic reflex in the dorsal horn that inadvertently activates the ventral roots (VR neural fibers), which include motor neural fibers leading to corresponding muscles. While DC nerve fibers are the intended targets in conventional SCS, in fact, the DR nerve fibers often are recruited first because of geometric, anatomical, and physiological reasons.
For example, the DR nerve fibers have larger diameters than the largest nearby DC nerve fibers, and thus, have a lower threshold at which they are excited. Other factors that contribute to the lower threshold needed to excite DR nerve fibers are the different orientations of the DC nerve fibers and DR nerve fibers, the curved shape of the DR nerve fibers, and the inhomogeneity and anisotropy of the surrounding medium at the entrance of the DR nerve fibers into the spinal cord. Thus, DR nerve fibers may still generate APs at lower voltages than will nearby DC nerve fibers. As a result, the DC nerve fibers that are desired to be stimulated have a lower probability to be stimulated than do the DR nerve fibers, and thus, the DR nerve fibers are often recruited, thereby potentially creating a monosynaptic reflect in the dorsal horn that leads to discomfort or muscle twitching, ultimately preventing satisfactory pain relief.
For reasons such as these, it is often desirable to modify the threshold at which neural tissue is activated in a manner that maximizes excitation of the target neural tissue, while minimizing excitation of the non-target neural tissue; that is, to increase the DR/DC fiber threshold ratio. This can be accomplished by medial-laterally aligning an electrode array (i.e., the electrodes are arranged transversely to the neural fibers of the spinal cord), and controlling the shape of the electric field generating activation region of the spinal cord in order to prevent the generation of APs in non-target neural fibers. In particular, an electrical pulse is sunk to a cathodic electrode located at the center of the spinal cord to depolarize the target tissue adjacent the cathodic electrode, thereby creating APs along the DC nerve fibers, while an electrical pulse is sourced to anodic electrodes on both sides of the cathodic electrode to hyperpolarize non-target tissue adjacent the anodic electrodes, thereby increasing the threshold of the DR nerve fibers.
While the stimulation of the spinal cord using a medial-lateral arranged electrode array in this manner has been shown to provide effective pain treatment, this approach does require multiple contacts in the medial-lateral direction, which, given a fixed number of electrodes, decreases the rostro-caudal span of the electrode array along the spinal cord. Furthermore, due to current shunting resulting from the close spacing between anodes required to achieve the selective stimulation field, high stimulation energy thresholds are typically required for medial-lateral electrode arrangements.
There, thus, remains to alternative means for increasing the DR/DC fiber threshold ratio.